This invention relates generally to the field of fluid-solid contacting. More specifically, this invention deals with the mixing of fluids between beds of particulate material. Included within the scope of this invention is the mixing of single phase or two phase fluids.
Fluid-solid contacting devices have a wide variety of applications. Such devices find common application in processes for hydrocarbon conversion and adsorption columns for separation of fluid components. When the fluid-solid contacting device is an adsorption column the particulate material will comprise an adsorbent through which the fluid passes. In the case of hydrocarbon conversion the fluid-solid contacting apparatus is typically a reactor containing catalyst. Typical hydrocarbon conversion reactions that may be carried out are hydrogenation, hydrotreating, hydrocracking, hydrodealkyalation.
Fluid-solid contacting devices to which this invention apply are arranged as an elongated cylinder having a vertical orientation through which an essentially vertical flow of fluid is maintained. Particulate material contained in this vessel is arranged in a series of vertically spaced beds. Fluids enters the vessel through at least one inlet and outlet located at opposing ends. The fluid can flow through the reactor in a upflow or downflow fashion. It is also commonly known to add or withdraw fluid from between the particulate beds. This is commonly done in adsorption schemes where the composition to the fluid passing between particle beds is changing or in hydrocarbon conversion processes where a quench system is used to cool fluid as it passes between beds.
Changes in the composition or properties of the fluid passing through the particular zone present little problem provided these changes occur uniformly. In adsorption systems these changes are the result of retention or displacement of fluids within the adsorbent. For reaction systems changes in temperature as well as composition of the fluid are caused by the particulate catalyst material contained in the beds.
Nonuniform flow of fluid through these beds can be caused by poor initial mixing of the fluid entering the bed or variations in flow resistance across the particulate bed. Variations in the flow resistance across the bed can vary contact time of the fluid within the particles thereby resulting in uneven reactions or adsorption of the fluid stream passing through the bed. In extreme instances this is referred to as channeling wherein fluid over a limited portion of the bed is allowed to move in a narrow open area with virtually no resistance to flow. When channeling occurs a portion of the fluid passing through the bed will have minimal contact with the particulate matter of the bed. If the process is one of adsorption the fluid passing through the channel area will not be adsorbed, thereby altering the composition of this fluid with respect to fluid passing through other portions of the absorbent bed. For a catalytic reaction the reduction in catalyst contact time will also alter the product composition of fluid as it leaves different portions of the catalyst bed.
In addition to problems of a fluid composition, irregularities in the particulate bed can also affect the density and temperature of the fluid passing through the bed. For many separations processe retained and displaced components of the fluid have different densities which tend to disrupt the flow profile through the bed. Nonuniform contacting with the adsorbent particles will exacerbate the problem by introducing more variation in the density of the fluid across the bed thereby further disrupting the flow profile of the fluid as it passes through the particle bed.
In reaction zones temperature variations are most often associated with nonuniform catalyst contact due to the endothermic or exothermic nature of such systems. Nonuniform contact with the catalyst will adversely affect the reaction taking place by overheating or overcooling the reactants. This problem is most severe in exothermic reactions where the higher temperature can cause further reaction of feed stock or other fluid components into undesirable products or can introduce local hot spots that will cause damage to the catalyst and/or mechanical components.
Therefore, in order to minimize the problems that are associated with variations in fluid flow through beds of particulate material, methods of remixing fluid between beds of catalyst or adsorbents have been incorporated into a number of processes. Devices for collecting and remixing a portion of the fluid moving through a series of particle beds are shown in U.S. Pat. Nos. 3,652,450 and 4,087,252. In these references, the remixing of the fluid is done in conjunction with the addition of a second fluid into the mixing zone between beds. In both of these references, mixing of the fluid passing between beds and the added fluid is performed in a number of discrete mixing chambers located in or between the lower boundary of the upper bed and the upper boundary of the lower bed.
U.S. Pat. No. 3,824,080 by Smith reveals an internal reactor configuration for mixing fluid passing between beds independent of a second added fluid in that zone. The Smith device collects a mixed phase fluid flow in a region between particle beds having a horizontal baffle containing a central opening for passing the fluid between beds. This central opening has a flow diverter device which directs all vapor flow through the top of the chamber and all liquid flow in through the sides. In the Smith invention, vapor and liquid impinge upon each other at right angles thereby effecting remixing. After the remixed vapor and liquid passes through the opening in the baffle it contacts another horizontal series of baffles for providing an even flow of fluid over the downstream particle bed.
U.S. Pat. No. 3,598,541 by Hennemuth et al. teaches the remixing of fluid passing between beds of particulate materials by direct impingement with a quench fluid added to the mixing zone. Mixing occurs in a centralized space through which all fluid passes. The centralized space contains an annular area defined by two vertically oriented cylinders. Fluid passing between beds enters via horizontally projecting holes in the outer cylinder, while the quench fluid enters through horizontally projecting holes in the inner cylinder. The lower end of the annular mixing zone communicates with the downstream particle zone to allow passage of the mixed fluid.
An object of the invention disclosed herein is to improve the mixing of fluids passing between beds of particulant material. It is a further object of this invention to achieve mixing of the fluid passing between beds independent of the addition of a second fluid into the zone between particle beds. A further object of this invention is to provide a simplified device for achieving mixing of fluid between beds which is easily incorporated into a minimal space between particle beds.